Some dumb idea I've been thinking in the last few days.
The scenario in particular takes an Abbasid Revolution that is suppressed by the ruling Umayyad Caliphate but the revolt cause enough noise that the Caliphate eastern provinces in great unrest as the multiple anti-Umayyad partisans are now leaderless but not totally destroyed.
So, could in this scenario the Dabuyid dynasty in Tabarestan exploit this turmoil in the Caliphate (by the time of Abbasid Revolution the Umayyads were either fighting or pacifying rebel regions though all of its territory) and launch a revolt that evolves in a full-blown conquest of Iran and Khorasan? Could this go as far as conquering Iraq and Jazira (kinda stretchy but eh)?
 
With no Abbasid takeover of the Umayyad Caliphate, Tang dynasty China most likely continues expanding into Central Asia uninterrupted, and might even end up vassalizing the aforemented Dabuyid Persia if such expansion is not interrupted by an alternate An Lushan Rebellion.
Now, as for post-reconquista Zoroastrian Persia, i think the Dabuyids will have some trouble ruling over their restored territory, considering the influence that Muslim proselytizers will have from there on in ("we took these lands for Dar-Al-Islam once, so why not try again?").
 
With no Abbasid takeover of the Umayyad Caliphate, Tang dynasty China most likely continues expanding into Central Asia uninterrupted
Although this newly established Iranian state wouldn't have the means (or priorities) for checking the Tang in Transoxiania it is important to note that the Tang Emperor didn't even knew that Talas was a thing for a while, that shows that Tang expansion in Central Asia was reaching this limits by the late 8th century, you could have Sinified (rather than Islamized) Turkic confederations in Central Asia, but a never-stopping Tang blob will probably end on the Syr Darya.
alternate An Lushan Rebellion.
Most likely will happen, An Lushan wasn't some great man, he was the symptom of a grave problem in the Tang empire, even if the man himself doesn't rise up another charismatic general probably will.
considering the influence that Muslim proselytizers will have from there on in
That's something interesting honestly, Al-Hajjaj ibn Yusuf the Umayyad governor of the Eastern provinces is reputed of killing Zoroastrian priests and burning all religious texts, the existence of surviving literature and clergy today in those bands proves that those claims are hyperbolic, but the harsh persecution may have to lead to multiple syncretic Islamo-Zoroastrian movements though Iran and Khorasan like Behafarid and the Khurramites, in the advent of a resurgent Mazdean Persia conquering those territories we could have several Islamic elements added to Zoroastrian orthodoxy, even if unwitting by part of the restored Zoroastrian clergy, maybe even Mazdakite elements (the Dabuyids came from Tabarestan, which is part of of the Azerbaijan macro-region that gave rise of the Neomazdakite Khurramite movement).
("we took these lands for Dar-Al-Islam once, so why not try again?").
Well, it'll depend on the policy of the Caliphate (Umayyad or a new dynasty) and how big they lost (ie Iraq and Jazira), we may see the Caliphate leaving Iran alone and becoming more Mediterranean focused, or renewed warfare.
 
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